实现点击Form1的send按钮则打开Form2，并将Form1文本框的值传给Form2。点击Form2的OK按钮则关闭Form2并将文本框的值传回Form1。

点击send打开Form2并传值有多种方式
委托传值
可以在new Form2时将值传给Form2，这时需要更改Form2的构造函数来接收，由于在Form2中不能调用Form1的控件，所以更改Form1的文本框的方法写在Form1里，用委托传给Form2，也是更改Form2的构造函数来实现。

代码如下：

namespace WindowsFormsApplication3
{
   //定义全局的委托
    public delegate void changeTextDelegate(string message);
    public partial class Form1 : Form
    {

        public Form1()
        {
            InitializeComponent();                    
        }

        private void button1_Click(object sender, EventArgs e)
        {
            //构造form2时将值和方法传给form2
            Form2 form2 = new Form2(textBox1.Text,changeText);
            form2.Show();

        }
        private void changeText(string message)
        {
            textBox1.Text = message;
        }
    }
}

namespace WindowsFormsApplication3
{

    public partial class Form2 : Form
    {
        public Form2()
        {
            InitializeComponent();
        }

        /// <summary>
        /// 重载Form2构造函数
        /// </summary>
        /// <param name="message">接收值</param>
        /// <param name="changeText">委托接收方法</param>
        public Form2(string message, changeTextDelegate changeText) : this()
        {
            textBox1.Text = message;
            //将更改Form1文本框值的方法赋给Form2私有的委托
            _changeText = changeText;
        }
        private changeTextDelegate _changeText;

        private void button1_Click(object sender, EventArgs e)
        {
            _changeText(textBox1.Text);
            this.Close();
        }
    }
}

事件传值
跟委托差不多

namespace WindowsFormsApplication3
{
    //定义全局的委托
    public delegate void changeTextDelegate(string message);
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            //构造form2时将值和方法传给form2
            Form2 form2 = new Form2(textBox1.Text);
            form2.changeTextEvent += changeText;
            form2.Show();

        }
        private void changeText(string message)
        {
            textBox1.Text = message;
        }
    }
}

namespace WindowsFormsApplication3
{

    public partial class Form2 : Form
    {
        public Form2()
        {
            InitializeComponent();
        }

        public Form2(string message) : this()
        {
            textBox1.Text = message;

        }
        public event changeTextDelegate changeTextEvent;

        private void button1_Click(object sender, EventArgs e)
        {
            changeTextEvent(textBox1.Text);
            this.Close();
        }
    }
}

用Owner属性

namespace WindowsFormsApplication3
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();                    
        }
        private void button1_Click(object sender, EventArgs e)
        {
            Form2 form2 = new Form2(textBox1.Text);       
            form2.Show(this);         
        }
        public void changeText(string message)
        {
            textBox1.Text = message;
        }
    }
}

namespace WindowsFormsApplication3
{
    public partial class Form2 : Form
    {
        public Form2()
        {
            InitializeComponent();
        }
        public Form2(string message) : this()
        {
            textBox1.Text = message;

        }
        private void button1_Click(object sender, EventArgs e)
        {
            Form1 form1 = (Form1)this.Owner;
            form1.changeText(textBox1.Text);
            //用fom1的Controls集合也可以
            // form1.Controls["textBox1"].Text = textBox1.Text;
            this.Close();
        }
    }
}

封装属性实时多次传值
上面的方法Form1只能传一次值给Form2.
用属性封装也能完成窗体传值，而且可以不关闭Form2，Form1多次传值给Form2,即可以实时传值。
把Form1的send按钮方法改成

        private void button1_Click(object sender, EventArgs e)
        {        
            if (form2 == null||form2.IsDisposed)
            {               
                form2 = new Form2();
                form2.textValue = textBox1.Text;
                form2.Show(this);
            }
            else
            {
                form2.textValue = textBox1.Text;
            }          
        }

在Form2里加属性

        public string textValue
        {
            get { return textBox1.Text; }
            set { textBox1.Text = value; }
        }

就可以了